1. Field of the Invention
This invention relates to cogeneration cycles for reheat turbines, and more particularly, to means for adjusting the temperature and pressure of steam extracted from a turbine power cycle having a steam reheater.
2. Description of the Prior Art
Cogeneration applications in which process steam is removed from a power generation turbine cycle has become increasingly popular as energy has become increasingly scarce and expensive. Cogeneration applications have typically been applied to steam turbine cycles having no reheat of the steam during its expansion through the turbine.
Power generation turbines usually constitute multiple fluidly communicating sections or cylinders. Steam reheaters are most frequently arranged to receive steam from a first turbine section exhaust port and discharge the reheated steam to a second turbine section inlet port. Process steam requirements often include pressures in the range of 400-700 psi. When reheat turbines are used, the first or high pressure turbine section usually exhausts at a pressure of 520-600 psi rated turbine load for throttle pressures in the range of 1800-2400 psi. Additionally, the steam pressure at the second or intermediate pressure turbine section inlet falls within the range of 470-540 psi at rated turbine load. The aforementioned steam pressure ranges have been found to constitute thermodynamically optimum values for reheat turbines. As can be seen, the high pressure turbine section exhaust and intermediate pressure turbine section inlet respectively have typical pressures which fall very close to the usual 700 and 400 psi process steam requirements. Additionally, extraction flow from the turbine causes a drop in the extraction pressure so as to necessitate integral steam chests and valves just after or downstream from the extraction points so as to maintain the desired extraction pressure for the process steam. Inclusion of such integral steam chests and valves increases the turbine length, complexity and cost while concomitantly reducing the turbine efficiency due to the inherent pressure losses when the extraction flow requirement to the process is zero. As such, providing cogeneration with a reheat turbine cycle has, in the past, required a more complex and costly turbine whose efficiency was reduced when the process steam flow requirement was zero.